1. Field of the Invention
The present invention relates to an antenna device used for an onboard millimeter wave radar, in particular, to a digital beam forming (DBF) radar that monitors the direction of travel of an automobile.
2. Description of the Related Art
A DBF radar includes a receiving array composed of a plurality of receiving antenna elements arranged at predetermined intervals (typically, regular intervals) in a scan direction. The DBF radar converts received signals from each receiving antenna element into digital data, performs arithmetic processing on the digital data to impart a phase shift to each received signal, and synthesizes the phase-shifted received signal to generate an equivalent scan beam. A method of direct azimuth detection, such as monopulse angle measurement, or a high resolution detection method, such as multiple signal classification (MUSIC) method, can also be applied. The DBF radar can scan at high speed with high precision without the need for any drive part or movable mechanism and therefore is widely used as the onboard millimeter wave radar.
However, the DBF radar requires performing measurement to prevent erroneous detection due to the grating lobe phenomenon.
FIG. 11 is a diagram illustrating an overview of the principle of azimuth detection using a receiving antenna array and reference symbols or the like used in the following description. A plurality of receiving antenna elements R0, R1, R2 and so on are arranged at regular interval P in the horizontal direction to form a receiving array. Each receiving antenna element includes an antenna and a receiver and an analog-to-digital signal converter connected to the antenna, although this drawing shows only the arrangement of the antennas. A coordinate system the X axis of which extends in the horizontal direction and the Z axis of which extends in a direction straight ahead of an aperture plane of the antenna is defined, and the XZ plane is a scan plane. The angle of deviation from the Z axis in the horizontal direction is denoted by θ, and in this drawing, the deviation to the right is denoted as a positive value (+), and the deviation to the left is denoted as a negative value (−).
For an incoming wave from the θ direction, a propagation path length difference ΔL occurs between the incident waves to adjacent receiving antenna elements, and there is a phase difference Δϕ between the received waves.ΔL=P·sin θ  Expression 1Δϕ=k·ΔL+2iπ  Expression 2where i denotes an integer (0, ±1, . . . ) that minimizes the absolute value of Δϕ, k denotes a wave number (=2π/λ), and λ denotes a wavelength in free space, which is 3.92 mm at 76.5 Ghz used for the onboard millimeter wave radar. According to these relations, the detection value Θ of the incoming wave azimuth is calculated from the phase difference.
                    Θ        =                  sin          -                      1            ⁢                          (                              Δϕ                kP                            )                                                          Expression        ⁢                                  ⁢        3            If Δϕ falls within a range of 0±π (180°), Θ agrees with θ, and the azimuth can be determined.The incoming wave azimuths for which Δϕ=π and 2π are denoted as χ and γ, respectively.
                    χ        =                              sin                          -              1                                ⁡                      (                          λ                              2                ⁢                                                                  ⁢                P                                      )                                              Expression        ⁢                                  ⁢        4                                γ        =                              sin                          -              1                                ⁡                      (                          λ              P                        )                                              Expression        ⁢                                  ⁢        5            When θ falls within a range of ±χ, azimuth detection can be achieved. For purposes of explanation, this range will be referred to as a main region, and the remaining range will be referred to as an outside region. In the outside region, if θ is slightly greater than χ (θ=γ+δ), the following relation holds, and left-to-right inversion occurs.Θ≈−δ  Expression 6If Θ is close to γ (θ=γ±δ), the following relation holds, and an object to the side is detected as being present in the front direction.Θ≈±δ  Expression 7To deal with a plurality of incoming waves, the number of receiving antenna elements needs to be increased according to the number of the incoming waves, and various separation methods need to be used. However, the detection region determined by the interval P and the relationship between the incoming wave azimuth θ and the detection value Θ still hold.
That is, erroneous detection occurs due to an incoming wave from the outside region. To prevent this, there are contemplated methods of reducing gain in the outside region of the radar antenna unit.
Japanese Patent Laid-Open No. 2013-032979 discloses an antenna arrangement that includes a slotted waveguide array and rectangular horns added thereto as a radiator. In this example, a radiation pattern of the horns, in particular, null characteristics, is used.
Japanese Patent No. 5667887 discloses radiating elements of a transmitting antenna are sequentially displaced in either lateral direction (scan direction) so as to be arranged in symmetry in a longitudinal direction.
The antenna disclosed in Japanese Patent Publication No. 2013-032979 can achieve high gain since the power supply loss in the waveguide is low, and the rectangular horns, which are highly efficient, are used. In addition, since the whole of the antenna is made of metal plates, significant performance variation, deformation or the like due to heat does not occur, and the antenna has a heat radiation effect. That is, the antenna has characteristics suitable for application to a small onboard radar. However, in order to achieve a desired directivity, more specifically, in order to achieve a directivity that attains high gain only in the main region and minimizes a side lobe in the outside region, by improving the radiation characteristics of the horns themselves, the dimensions, in particular, the depth, of the horns need to be increased. Thus, this antenna has an undesirably large size. In view of such circumstances, the structure in which the horns are displaced from each other in the lateral direction, such as that disclosed in Japanese Patent No. 5667887, has been devised. However, Japanese Patent No. 5667887 discloses only the number of radiating elements of a printed antenna and an optimum arrangement depending on the intervals between the elements.